Tomb Nightmare is protected by Flame Geysers, you need to trigger the 3 traps with bone horrors, otherwise you will need to kill your self twice to get there (a devourer triggers the first one), or have a significant amount of health to absorb the damage (upwards of 300).

The Tomb Nightmare has VERY low armor versus cold damage, resulting in an easy kill via Deathly Swarm, obtained at the beginning of the quest.